1. Technical Field
The present invention relates to an evaluation method of a perpendicular magnetic recording medium and a manufacturing method of a perpendicular magnetic recording medium to be mounted on a magnetic disk device such as a perpendicular magnetic recording type HDD (hard-disk drive).
2. Background Art
With the recent trend to higher-capacity information processing, various information recording technologies have been developed. Particularly, a surface recording density of an HDD (hard-disk drive) using the magnetic recording technology has continuously increased by the rate of approximately 100% a year. In recent years, an information recording capacity exceeding 250 G bytes per disk is required for a magnetic disk having a diameter of 2.5 inches used in HDD or the like, and in order to meet such demand, realization of an information recording density exceeding 400 Gbits per 1 square inch is in demand. In order to achieve the high recording density in a magnetic disk used in an HDD or the like, magnetic crystal grains constituting a magnetic recording layer handling recording of an information signal need to be refined, and its layer thickness needs to be reduced at the same time. However, in the case of a prior-art magnetic disk of an in-plane magnetic recording method (also referred to as longitudinal magnetic recording method or horizontal magnetic recording method), development of the refining of the magnetic crystal grains would result in degradation of the thermal stability of the recording signal by superparamagnetic phenomenon. This generates a thermal fluctuation phenomenon causing the recording signal to disappear, which interrupts an increase in recording density of the magnetic disk.
In order to solve this obstructive factor, a magnetic disk of a perpendicular magnetic recording type has been proposed in recent years. In the case of the perpendicular magnetic recording method, unlike the in-plane magnetic recording method, a magnetization easy axis of a magnetic recording layer is adjusted to be oriented in the perpendicular direction with respect to a substrate surface. As compared with the in-plane recording method, the perpendicular magnetic recording method can suppress the thermal fluctuation phenomenon, and this is suitable for higher recording density. A technology relating to a perpendicular magnetic recording medium formed of a soft magnetic layer, an underlayer, a Co perpendicular magnetic recording layer, a protective layer and the like on a substrate in this order is disclosed in Japanese Unexamined Patent Application Publication No. 2002-92865, for example. Moreover, U.S. Pat. No. 6,468,670 discloses a perpendicular magnetic recording medium having a structure in which an artificial lattice film continuous layer (exchange coupling layer) exchange-coupled to a particle recording layer is made to adhere.
At present, higher recording density in the perpendicular magnetic recording medium is in demand.
In order to further improve the recording density of the magnetic recording medium under these circumstances, both a linear recording density (BPI: Bit Per Inch) in the circumferential direction and a track recording density (TPI: Track Per Inch) in the radial direction need to be improved.
In a magnetic recording reproducing head which records/reproduces a signal with respect to such a magnetic recording medium, recording and reproduction is beginning to be performed by separate heads with the trend to a higher density of the magnetic recording technology. Therefore, as illustrated in FIG. 8, a recording head 20 such as a single magnetic polar head, trailing sealed head or the like and a reproduction head 22 such as a large-sized magnetic resistance type (GMR) head, a tunnel magnetic resistance effect type (TuMR) head or the like are arranged separately.
These separate recording head 20 and reproduction head 22 are arranged linearly on a slider, but since the tracks for recording and reproduction are formed circumferentially along the magnetic recording medium, the reproduction head 22 needs to be offset by approximately 169 nm at the maximum, for example, to the inner periphery side in the radial direction with respect to the recording head 20 in order to arrange the recording head 20 and the reproduction head 22 on the tracks.
By referring to FIG. 9, the recording head 20 and the reproduction head 22 placed on the extension line in the longitudinal direction of a suspension 26 are not moved for offset and they are located on separate tracks 30 and 32 on a magnetic recording medium 28, respectively, in the slider 24. Therefore, the reproduction head 22 needs to move from the track 32 to the track 30 with the offset of a predetermined amount 40 in order to reproduce a signal recorded in the track 30 by the recording head 20.
A value of the offset of the reproduction head 22 is acquired through recording/reproduction of an actual signal. For example, the magnetic recording medium is rotated, a signal is recorded in a predetermined on-track position from the recording head 20, and then, the reproduction head 22 is moved, and a position where a reproduction output of the recorded signal becomes the maximum is searched. A movement amount of the reproduction head 22 for this search becomes the value of the offset. This offset is stored in a magnetic disk device, and the recorded signal is accurately reproduced by moving the reproduction head in advance for the stored offset for the next reproduction.
However, set offset might be shifted from actual offset due to a search error of a position where the output signal becomes the maximum, drift of the offset caused by a temperature change of the magnetic disk device and elapse of time and the like. Since a track interval was large and a recordable width in the radial direction was wide in a prior-art medium which has a small recording density, such a small error in the offset was allowed.
However, an influence of such an offset error cannot be ignored in the recent perpendicular magnetic recording medium having a high recording density. For example, recording is made at a position other than the memory region (recordable width) due to the offset error in the magnetic recording medium having a small recordable width, and a signal might be buried in noises and the reproduction head might not be able to identify the signal. Therefore, it becomes also necessary to ensure the recordable width in the radial direction as much as possible while the track recording density is improved.
In order to estimate the recordable width of the magnetic recording medium, a technology is known that an off-track signal different from the on-track one is intentionally recorded on the both sides of the on-track one, a limit position where the on-track signal can be identified from the off-track signal is detected, and this limit position is derived as an off-track margin (Patent Document 1).
However, in such a technology of deriving an off-track margin, a border line with the off-track signal intentionally recorded in a region adjacent to the track is merely derived and the influence of noise (leakage magnetic field) from the adjacent track cannot be measured.
The necessity to ensure some recordable width has been described, but if the recordable width is merely to be ensured in the trend to higher density of the track recording density TPI, the whole track width becomes large, and an influence to the adjacent track is increased. If the influence is large, the recorded contents of the adjacent track might be deleted or its reproduction output might become unstable. Therefore, a magnetic disk having a small influence on the adjacent track is desirable while the recordable width is ensured.